The pulmonary circulation differs in certain important respects from the systemit circulation. The pulmonary arteries are comparatively thin-walled and the muscular coat, even in the finer branches, is hut slightly developed. Hence there is little of that variation in blood-pressure which in the systemic circulation is brought about by alterations in the state of contraction of the arterioles. The pulmonary blood-pressure is normally low and is subject to little variation, except in a purely passive manner. It might be supposed that the pulmonary arterial pressure would correspond and vary with the systemic arterial pressure, but it does so to a very slight extent. The mitral valve when it is sufficient seems to cut off the systemic from the pulmonary circulation, so that there may be great elevation of the systemic blood-pressure whilst the pulmonary pressure is only slightly raised or even remains normal. Thus in asphyxia, where the systemic pressure is greatly elevated, the pulmonary is comparatively little affected. It follows from these facts, however, that pathological variations in the pulmonary circulation such as that produced by insufficiency of the mitral valve will tell greatly, as the means of compensation which the systemic arteries possess are much less efficiently present in the pulmonary arteries.